Polymeric electro-optic (EO) materials have demonstrated enormous potential for core application in a broad range of systems and devices, including phased array radar, satellite and fiber telecommunications, cable television (CATV), optical gyroscopes for application in aerial and missile guidance, electronic counter measure systems (ECM) systems, backplane interconnects for high-speed computation, ultrafast analog-to-digital conversion, land mine detection, radio frequency photonics, spatial light modulation and all-optical (light-switching-light) signal processing.
Nonlinear optic materials are capable of varying their first-, second-, third- and higher-order polarizabilities in the presence of an externally applied electric field or incident light (two-photon absorption). In telecommunication applications, the second-order polarizability (hyperpolarizability or β) and third-order polarizability (second-order hyperpolarizability or γ) are currently of great interest. The hyperpolarizability is related to the change of a NLO material's refractive index in response to application of an electric field. The second-order hyperpolarizability is related to the change of refractive index in response to photonic absorbance and thus is relevant to all-optical signal processing. A more complete discussion of nonlinear optical materials may be found in D. S. Chemla and J. Zyss, Nonlinear optical properties of organic molecules and crystals, Academic Press, 1987 and K.-S. Lee, Polymers for Photonics Applications I, Springer 2002.
Many NLO molecules (chromophores) have been synthesized that exhibit high molecular electro-optic properties. The product of the molecular dipole moment (μ) and hyperpolarizability (β) is often used as a measure of molecular electro-optic performance due to the dipole's involvement in material processing. One chromophore originally evaluated for its extraordinary NLO properties by Bell Labs in the 1960s, Disperse Red (DR), exhibits an electro-optic coefficient μβ˜580×10−48 esu. Current molecular designs, including FTC, CLD and GLD, exhibit μβ values in excess of 10,000×10−48 esu. See Dalton et al., “New Class of High Hyperpolarizability Organic Chromophores and Process for Synthesizing the Same”, WO 00/09613.
Nevertheless extreme difficulties have been encountered translating microscopic molecular hyperpolarizabilities (β) into macroscopic material hyperpolarizabilities (X(2)). Molecular subcomponents (chromophores) must be integrated into NLO materials that exhibit: (i) a high degree of macroscopic nonlinearity; and, (ii) sufficient temporal, thermal, chemical and photochemical stability. Simultaneous solution of these dual issues is regarded as the final impediment in the broad commercialization of EO polymers in numerous government and commercial devices and systems.
The production of high material hyperpolarizabilities (X(2)) is limited by the poor social character of NLO chromophores. Commercially viable materials must incorporate chromophores with the requisite molecular moment statistically oriented along a single material axis. In order to achieve such an organization, the charge transfer (dipolar) character of NLO chromophores is commonly exploited through the application of an external electric field during material processing which creates a localized lower-energy condition favoring noncentrosymmetric order. Unfortunately, at even moderate chromophore densities, molecules form multi-molecular dipolarly-bound (tentrosymmetric) aggregates that cannot be dismantled via realistic field energies. As a result, NLO material performance tends to decrease dramatically after approximately 20-30% weight loading. One possible solution to this situation is the production of higher performance chromophores that can produce the desired hyperpolar character at significantly lower molar concentrations.
Attempts at fabricating higher performance NLO chromophores have largely failed due to the nature of the molecular architecture employed throughout the scientific community. Currently all high-performance chromophores (e.g., CLD, FTC, GLD, etc.) incorporate protracted “naked” chains of alternating single-double π-conjugated covalent bonds. Researchers such as Dr. Seth Marder have provided profound and detailed studies regarding the quantum mechanical function of such “bond-alternating” systems which have been invaluable to our current understanding of the origins of the NLO phenomenon and have in turn guided present-day chemical engineering efforts. Although increasing the length of these chains generally improves NLO character, once these chains exceed ˜2 nm, little or no improvement in material performance has been recorded. Presumably this is largely due to: (i) bending and rotation of the conjugated atomic chains which disrupts the π-conduction of the system and thus reduces the resultant NLO character; and, (ii) the inability of such large molecular systems to orient within the material matrix during poling processes due to environmental steric inhibition. Thus, future chromophore architectures must exhibit two important characteristic: (i) a high degree of rigidity, and (ii) smaller conjugative systems that concentrate NLO activity within more compact molecular dimensions.
Long-term thermal, chemical and photochemical stability is the single most important issues in the construction of effective NLO materials. Material instability is in large part the result of three factors: (i) the increased susceptibility to nucleophilic attack of NLO chromophores due to molecular and/or intramolecular (CT) charge transfer or (quasi)-polarization, either due to high-field poling processes or photonic absorption at molecular and intramolecular resonant energies; (ii) molecular motion due to photo-induced cis-trans isomerization which aids in the reorientation of molecules into performance-detrimental centrosymmetric configurations over time; and (iii) the extreme difficulty in incorporating NLO chromophores into a holistic cross-linked polymer matrix due to inherent reactivity of naked alternating-bond chromophore architectures. Thus, future chromophore architectures: (i) must exhibit improved CT and/or quasi-polar state stability; (ii) must not incorporate structures that undergo photo-induced cis-trans isomerization; and (iii) must be highly resistant to polymerization processes through the possible full-exclusion of naked alternating bonds.
The present invention seeks to fulfill these needs through the innovation of fully heterocyclical anti-aromatic chromophore design. The heterocyclical systems described herein do not incorporate naked bond-alternating chains that are susceptible to bending or rotation. The central anti-aromatic conductor “pull” the molecule into a quasi-CT state; since aromaticity and non-CT states are both favorably low-energy conditions, charge transfer and aromaticity within the molecular systems described herein are set against each other within a competitive theater. This competitive situation is known as CAPP engineering or Charge-Aromaticity Push-Pull. As a result, the incorporation of anti-aromatic systems dramatically improves the conductive properties of the central π-conjugated bridge providing for smaller molecular lengths with significantly greater NLO property. Because all the systems described, herein are aromatic in their CT state and quasi-aromatic in their intermediate quasi-polarized states, this structure is expected to dramatically improve polar-state stability. Furthermore, novel electronic acceptor systems are described herein which are expected to significantly improve excited-state and quasi-CT delocalization making the overall systems less susceptible to nucleophilic attack. The heterocyclical nature of all the systems described herein forbids the existence of photo-induced cis-trans isomerization which is suspected as a cause of both material and molecular degeneration. Finally, the invention provides for chromophoric systems that are devoid of naked alternating bonds that are reactive to polymerization conditions.